Weight-on-bit (hereinafter WOB) is generally recognized as being an important parameter in controlling the drilling of a well. The weight is applied to the bit by a string of heavy drill collars that is attached immediately above the bit and suspended in the borehole on smaller diameter drill pipe. In conventional drilling practice, the entire length of the drill pipe and an upper portion of the drill collar string are suspended at the surface from the derrick in tension, so that the amount of WOB can be varied by changing the indicated surface hookload. Properly controlled WOB is necessary to optimize the rate that the bit penetrates a particular type of earth formation, as well as the rate of bit wear. WOB also is utilized in controlling the direction of the hole, and accurate measurement thereof can be used in analyzing drilling rate "breaks" indicative of entry of the bit into more porous earth formations. Thus, it will be recognized that a precise and accurate measurement of the WOB parameter on an ongoing basis is extremely important in the drilling process.
Torque also is an important measure useful in estimating the degree of wear of the bit, particularly when considered together with measurements of WOB. Excessive torque is indicative of serious bit damage such as bearing failure and locked cones.
In the past, WOB and torque measurements have been made at the surface, the former by comparing indicated hookload weight to off-bottom weight of the drill string, and the latter by measurement of current supply to the rotary table electric drive motor, or mechanical force on the rotary drive chain idler. However, a surface measurement is not always reliable due to drag of the drill string on the borehole wall, and other factors. As a consequence, there has been a long-felt need to measure actual values of weight and torque at the bit itself to provide truly reliable information.
Recent developments in borehole telemetry systems have made it possible to make the measurements downhole, but for the most part, the downhole sensors that have been proposed are subject to significant inaccuracies due to the effects of well pressures and temperature transients that are present during the drilling process. In particular, those designs using an auxiliary member or sleeve to amplify the strain are believed to be inadequate because such auxiliary member cannot distinguish between strain due to weight and a strain due to thermal expansion, the latter being a very complex, time-dependent distribution of strain that can render the measurements, particular WOB, meaningless.
The general object of the present invention is to provide a new and improved apparatus for measuring WOB and/or torque downhole with high accuracy.
Another object of the present invention is to provide a sensor apparatus of the type described that employs strain gauges to measure axial and torsional forces on the bit in an improved manner.
Still another object of the present invention is to provide a new and improved WOB sub having strain gauge transducer means constructed and arranged to be substantially insensitive to stresses induced by changing temperatures and pressures during the drilling of a well.